Optical compensating filter with selective radial absorption distribution

ABSTRACT

THERE IS DISCLOSED AN OPTICAL FILTER, PRIMARILY FOR USE WITHIN A LASER RECORDER, WHICH OPERATES TO SELECTIVELY ABSORB PORTIONS OF A LIGHT BEAM OF NONUNIFORM INTENSITY PASSING THERETHROUGH SO AS TO RESULT IN THE EMERGENCE THEREFROM OF A LIGHT BEAM OF UNIFORM INTENSITY.

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United States Patent O OPTICAL COMPENSATING FILTER WITH SELEC- TIVERADIAL ABSORPTION DISTRIBUTION Kenneth C. Hudson, Philadelphia, Pa.,assignor to RCA Corporation, a corporation of Delaware Originalapplication Oct. 11, 1967, Ser. No. 674,614, iiow Patent No. 3,465,347,dated Sept. 2, 1969. Divided and this application Mai'. 27, 1969, Ser.No. 829,832

Int. Cl. G02b 5/22 U.S. Cl. 350-8 4 Claims ABSTRACT OF THE DISCLOSUREThere is disclosed an optical filter, primarily for use within a laserrecorder, which operates to selectively absorb 'portions of a light beamof nonuniform intensity passing therethrough so as to result in theemergence therefrom of a light beam of uniform intensity.

The invention herein described was made in the course of or under acontract or subcontract thereunder with the Department of the Air Force.

This is a division of application Ser. No. 674,614, filed Oct. 11, 1967,now U.S. Patent No. 3,465,347, September 2, 1969.

BACKGROUND OF THE INVENTION This invention relates to flying spotrecorders and, more particularly, to flying spot recorders which utilizea laser as the basic recording energy source.

To provide a maximum of recorded information per unit volume ofrecording media, it is necessary to use high packing density recordingtechniques. These techniques require a small high energy densityscanning spot which is capable of being modulated by the signal to berecorded. A recording spot in the order of 5.0 microns or less indiameter can be produced by a coherent radiation source and adiffraction limited optical system. With the advent of the laser, it hasbecome possible to produce a coherent beam of high intensity light whichis capable of being modulated and focused upon a photo-sensitive filmemulsion to produce a permanent record of processed signals. The opticalsystem used for focusing the recording spot must perform the followingfunctions:

1) Collect and focus sufficient radiation from the modulated radiationsource to expose the recording medium during the exposure time availablefor each recording bit;

(2) Produce an image on the recording medium that will have a spotdiameter in the order of microns or smaller;

(3) Provide sufficient working distance and clearance for the operationof the scanning mirror; and

(4) Provide uniform energy density in the spot as it scans therecord-ing medium.

SUMMARY OF THE INVENTION This invention relates to a novel opticalsystem which solves the problems inherent in focusing a modulated energysource into a high energy density recording spot and deflecting the spotacross a photosensitive film in a manner to permit the energy density toremain constant as the spot traverses the film.

The utilization of a recording spot of both high intensity and smalldiameter is particularly desirable because it permits the achievement ofa high storage density of recorded signals at an increased recordingspeed. It will be shown that to produce a recording spot of both highintensity and small diameter, utilizing prior art techniques, resultedin a large loss of laser power during 3,558,208 Patented Jan. 26, 1971transmission through the optical system, i.e., in the order of percent.The present invention permits the production of the desired recordingspot and uniform film exposure with a substantial increase in effciencyof laser power transmission. l

Accordingly it is an object of the present invention to provide anoptical system, primarily for use within a flying spot laser recorder,which provides a high energy density recording spot.

An additional object is to provide such an optical system wherein theuniformity of energy density of the recording spot is maintained as thespot traverses the recording medium.

Another object of the present invention is to provide DESCRIPTION OF THEFIGURES AND PREFERRED EMBODIMENT The present invention will be morefully understood when the following description is read in conjunctionwith the accompanying drawing wherein: l

FIG. 1 is a block a laser recorder;

FIG. 2 is a schematic diagram of a laser recorder as known in the priorart;

FIG. 3 is a generalized profile of a Gaussian distribution representingthe beam intensity across the output beam of a laser source;

FIG. 4 is a schematic diagram of a laser recorder in accordance with thepresent invention; and

F-IG. 5 is a cross sectional view of the filter shown in FIG. 4,superimposed on a'set of grid coordinates.

In the recording mode of operation of a flying spot recorder, the majorfunctions which must be implemented are:

(a) the establishment of a basic recording energy source;

(b) modulation of this energy source by the signals to be recorded;

(c) focusing of the modulated energy source into a high energy densityrecording spot; and

(d) scanning of a recording medium by this recording spot.

The laser has proven effectiveas the basic recording energy source sinceit is an extremely bright source consistent with wideband intensitymodulation techniques. Its energy is capable of being collected andformed into a recording spot approaching diffraction limited performanceat high efficiency. The intensity modulation of the laser isaccomplished through the application of electrooptic techniques.Wideband modulation techniques which make use of electro-optic crystalsprovide intensity modulation of the external laser source by applicationof a signal voltage. The recording spot is formed optically. In

the general case, the intensity modulated laser beam is Y diagram of thebasic components `of v sembly is generally used to effect scanning ofthe recording medium.

To reproduce the recorded signals, the developed film istransported-past an unmodulated scanning spot of coherent light. Thereadout spot energy is modulated by being passed through the -film. Whencollected and detected by a photosensitive device, the energy isconverted into an electrical signal which corresponds with the recordedsignal.

Turning now to a brief description of the recorder depicted by FIG. 1,the laser 100 provides a coherent light beam 102 of high intensity whichis directed into an intensity modulator 110. The modulator 110 issimultaneously provided with input signals 112 to be recorded. The inputsignals 112 cause the modulator 110 to intensely modulate the laserlight 102 in relation to the characteristics of the input signals 112.The modulated light 114 is focused into a high energy density recordingspot by the spot forming optics 120. The recording spot is thenreflected by an appropriately disposed scanning mechanism 130 onto therecording film 140 which is advanced by the film transport 150; thescanning mechanism 130 normally taking the form of a rotating polygonalmirror.

FIG. 2 shows an optical system for use in a laser recorder, inaccordance with the prior art. As shown therein, the optical system of alaser recorder may be described in terms of two component sub-systemsreferred to as the imaging optics and the beam-enlarging optics.

The imaging optics system is represented in FIG. 2 by the imaging lens240 and the polygonal scanning mirror 250. The modulated radiation 242emerging from the imaging lens 240 is focused thereby onto a recordingmedium represented in FIG. 2 by the focal surface 260. A mechanicalpolygonal scanning mirror 250 is interposed between the imaging lens 240and the recording medium 260 and, wheredesirable, a plane mirror (notshown) may be positioned between the imaging lens 240 and the scanningmirror 260 to defiect the image thereby permitting the scanning mirror250 and recording medium 260 to be favorably disposed.

The beam-enlarging optical system is represented in FIG. 2 by a firstlens 220 which enlarges the diameter of the modulated laser beam 215passing therethrough and a collimating lens 230 which receives theenlarged beam 22S and collimates it for transmission to the imaging lens240.

A laser beam emerging from a gas laser 200 generally has a diameter inthe order of .025 to .25D inch, depending on the particular laser used.The distribution of the beam intensity across the laser aperture isgenerally Gaussian, as shown in FIG. 3, with the beam diameter definedwhere the intensity has decreased to 1/e2 of its peak value.

In order for the imaging lens 240 to form an Airy disc, therebyresulting in a high energy density scanning spot, the entrance pupil ofthe lens must be uniformly filled with monochromatic radiation. If theenlargement of the beam 215 by the beam-enlarging optics 220, 230results in a collimated beam 235 of diameter less than the entranceaperture of the imaging lens 240, the distribution in the image willremain Gaussian rather than an Airy disc.

In the prior art system, as shown in FIG. 2, the irradiation across theaperture of the imaging lens 240 by an expanded laser beam 23S may berepresented by:

I=irradiation at center of beam p=radial distance from beam centera'=standard deviationof the expanded laser beam that if a ismade toequal pm/where p0 is the maximum (Gaussian) radial distance of the lens,then I(p) decreases by only 10 percent at the edge of the lens ascompared with I0.

It can be shown that the laser power through the lens is represented by:

(2) Power=Pt(l-eP2/2a'2) where Pt equals the total laser power output.It may similarly be shown that setting a' equal to pM/results in apercent diminution 0f the laser power transmitted through the lens.

The foregoing indicates that achieving a relatively uniform irradiation,utilizing prior art techniques, whereby the irradiation at the edge ofthe lens decreases by only l0 `percent as compared with the irradiationat the center of the lens, results in a 90 percent loss in laser powertransmitted through the lens as represented by the unused portions 227of the enlarged beam shown in FIG. 2. Furthermore, to accomplish thisresult, it is readily evident that a beam enlarger of relatively largesize is required to provide the necessary a.

FIG. 4 shows an optical system for use in a laser recorder inaccordance-with the present invention. It may be readily seen that thesystem of FIG. 4 differs from the system of FIG. 2 essentially as a.result of the inclusion of a filter 470 between the collimating lens 430and the imaging lens 440. Furthermore, as will be shown, thecharacteristics of the beam-enlarging optics subsystem are substantiallydifferent from those of the subsystem of FIG. 2.

In the beam-enlarging optics subsystem shown in FIG. 4, the lenscharacteristics are chosen so that a substantially greater fraction ofthe total laser radiation 425 is collimated by lens 430 and transmittedtherethrough. This results in a highly nonuniform irradiation across theexit pupil of the collimating lens 430, which is graphically representedin FIG. 3. The collimated beam 435 of nonuniform irradiation is thentransmitted through the filter 470 which operates to selectively absorbportions of the beam such that upon emergence therefrom the imaging lens440 is uniformly irradiated and therefore capable of transforming thedistribution of the beam 475 passing therethrough into an Airy disc.

It has been found that a filter designed in accordance with thefollowing teaehings'will accomplish the desired result, i.e., theemergence therefrom of a beam of uniform irradiation coupled with a moreefficient utilization of laser power, and permits the formation of arecording spot exhibiting uniform energy density as it scans therecording medium. Furthermore, the optical magnification required issignificantly decreased, thereby simplifying the design of thebeam-enlarging optical system. v

If, as shown in FIG. 4, a filter 470 is placed between the collimatinglens 430 and the imaging lens 440, the irradiation IL of the imaginglens 400 is represented by:

where T(p) :transmission characteristic` of the filter as a function ofp.

By designing the filter 470 such that which is a constant value at everypoint along the surface of the imaging lens 440; the value of p0 being aphysical dimension fixed by the lens aperture and a' depending on thebeam magnification.

The optimum value for a', which may be solved for by maximizing thepower to the aperture of the image lens 400 equals /V2. This results ina transmission of approximately 3.7 timesthe laser power through thelens when compared with the prior art as represented by FIG. 2.

For an optical absorbing medium of the type described, the transmissionis given by Beers Law:

(6) T: e-Bt where B=attenuation constant t: thickness.

@Ewa/2E Equation 7 is the equation of a parabola. The grid coordinatesin FIG. 5 have superimposed thereon a representation of this parabolawith the thickness (t) plotted as a function of radial distance (p)measured from the optical axis.

It may be seen that when p is equal to zero, t equals l/B; and when p isequal to p (which equals a V as an optimum condition, tequals zero.

To meet the design requirements previously set forth, the transmissionat the center of the filter (i.e., p=0) should be approximately 37percent of the transmission at the edge of the filter (i.e., p=p0=a\/2),where the transmission is theoretically 100 percent. As a practicalmatter 100 percent transmission is never attained due to reflections.These refiections can however, be minimized through the use ofanti-reflective coatings.

The absorbing material 572 and/or the index matching material 574 may bea liquid or a solid. Choosing an absorbing material 572 with arelatively high attenuation coeflicient (B) has two advantages; theabsorption of the index matching material 520 which varies withthickness can be neglected; and a spherical approximation can be madefor the parabola since the center thickness of the absorbing material572 becomes much smaller than the maximum radius (p0). A sphericalsurface is sgnilicantly easier to achieve in practice than a paraboloid.If a cover material 576 is required for containing the absorbingmaterial 572, its refractive index may be of any value since it willgenerally take the shape of a plane parallel disc. In practice it willusually be expedient to make the cover 576 of the same optical materialas the indexmatching material 574.

A representative list of absorbing materials, and their characteristics,is presented below:

While there has been shown and described herein a preferred embodimentof the invention, many changes will be readily apparent to those skilledin the art. For example, a filter of uniform thickness wherein theattenuation constant of the absorbing material is varied as a functionof p may be used; or any of a number of combinations of filterthicknesses and attenuation constants may be designed. Such changes areto be considered as being within the scope of the invention, and it isintended to limit same only as defined in the claims which follow.

' What is claimed is:

1. An optical filter capable of transforming a certain input light beamimpinging thereon, said certain input beam having an intensity whichvaries from point to point over its cross sectional area in accordancewith a predetermined Gaussian function, to an output light beam emergingtherefrom, said output beam having an intensity which is uniform frompoint to point over its cross section area,

said filter having a transmission characteristic T which varies frompoint to point over its cross sectional area as a function of the radialdistance p measured from its optical axis, in accordance with therelationship:

where e is the base of the natural logarithm system;

p0 is the maximum radial distance of said filter; and

a is the standard deviation of said predetermined Gaussian function ofsaid certain input beam.

2. An optical filter as defined in claim 1, said filter comprising ahomogeneous medium having a fixed attenuation coeiiicient B, thedimensional thickness t of said medium varying in a direction parallelto the light beam passing` therethrough as a function of the radialdistance p, measured from the optical axis of said filter, substantiallyin accordance with the relationship:

3. An optical system for transforming a modulated energy source suppliedthereto into a high energy density spot capable of being deflectedacross a focal surface wherein the energy density of the spot remainssubstantially constant as it traverses the focal surface,

said modulated energy source characterized by a beam of light having anintensity which varies from point to point over its cross sectional areain accordance with a predetermined Gaussian function, comprising:

(a) means for enlarging the cross sectional area of said modulatedenergy source;

said enlarged modulated energy source;

(c) an optical filter disposed to receive said selected portion of saidenlarged beam, said filter having a transmission characteristic whichvaries from point to point over its cross sectional area as a functionof the radial distance measured from its optical axis, said filterselectively absorbing portions of said enlarged modulated energy sourceto cause said enlarged modulated energy source to emerge from saidfilter with an intensity which is uniform from point to point over itscross sectional area; and

(d) imaging means disposed to receive said uniform intensity energysource and focus same into a high energy density spot.

4. An optical system as defined in claim 3 wherein the transmissioncharacteristic of said optical filter varies in accordance with therelationship where e is the base of the natural logarithm system;

p is the radial distance of said filter measured from its optical axis;

3,362,285 l/l968 Hora 33l-94.5X

DAVID SCHONBERG, Primary Examiner T. H. KUSMER, Assistant Examiner U.S.Cl. X.R.

(b) means for collimating a selected portion of

